Regulated power supplies have been a basic necessity in the electronics industry for many years. Early implementations achieved voltage regulation through the use of a series pass device that controlled the flow of current as a function of the demand of the load. This control was accomlished by sensing any change in output voltage caused by changing load requirements and feeding it back through analog control circuitry that could adjust the current flowing through the series pass device.
The major disadvantage of this early approach was that because the power supply operated at line frequency (60Hz) as the current rating of the supply increased, larger and larger iron transformers, filter capacitors, and iron chokes were required to handle the load requirements. In addition, the efficiency of power supplies based on the approach dropped significantly (as low as 20%) when low voltages were required.
Switching power supplies were developed to overcome these disadvantages. A typical switching power supply is shown and described in the manual entitled "JP Series Power Supplies, Technical Information", published by ACDC Electronics, Inc. In this approach, unregulated, high DC voltage (generally derived from a 110/220 volt, 60 Hz line) is chopped at a high frequency (typically, 20 Hz), stepped down through a ferrite transformer, and then rectified and filtered to obtain the desired DC output voltage. As in the earlier approach, regulation of the output voltage as load current varies is accomplished by feedback control using analog circuitry. Any deviation of the output voltage from its desired value produces an error signal. This signal is used to control the width of the pulses that chop the unregulated, high DC voltage. If the load current demand increases, the feedback control widens the pulse width thereby causing more energy to be transmitted through the transformer to the load. Conversely, the pulse width is narrowed if the load demand decreases.
The algorithm used for feedback control is almost always based upon proportional control. The error signal caused by a deviation in output voltage is compared against a voltage ramp. When the amplitude of the error signal and the voltage ramp are equal, the pulse is initiated. The pulse is always terminated at the end of the chopping cycle.
The main limitation of the analog implementation of this feedback control algorithm is that the switching power supply has limited ability to respond to large load current transients. Typically, a 50% change in load current produces a 5-10% change in output voltage before the control circuitry is able to return the output to within the regulation band. Furthermore, this recovery process usually requires a significant period of time. Power supply response to load transients can be improved by adding derivative control to the proportional control; however, this adds considerable complexity and cost to the power supply.
Analog feedback control systems have other disadvantages and limitations. First, they experience both short and long term stability problems because of component drift. Second, each output voltage to be regulated requires a dedicated analog control system. This means that power systems which produce multiple, independent, regulated output voltages must have an independent analog controller for each voltage. Since many contemporary applications require three to five independent voltages, duplicated analog circuitry contributed significantly to the cost of their power systems.
An ideal feedback control system would be one that could incorporate the advantages of proportional, derivative, and integral control for multiple, independent, regulated output voltages at low cost in a simple implementation that is relatively independent of component drift. The system would adapt to the needs of the external environment by using whatever control methods were appropriate for affecting a desired response. For example, it could use integral or proportional control when the load was relatively constant, and then instantly switch to derivative control when a transient occurs.